Celsius thermometer (attached to a barometer) made by J.G. Hasselström,
Stockholm, late 18th century.

History of the Celsius temperature scale

Anders Celsius early became engaged in the general problem of weights and measures,
including temperature measurements. Already as a student he assisted the astronomy
professor Erik Burman in meteorology observations. At that time there existed a
large variety of thermometers with different scales. Perhaps he already at this stage realized the necessity of a common international scale.

A temperature scale must be based on one or two standard temperatures,
called fixed points. For those it was natural to choose temperatures within the
temperature domain of practical interest, i.e. from about plus forty to minus
twenty in modern Celsius degrees. Thermometers were simply used in meteorology,
in horticulture, and sometimes for indoor use. As fixed points one could use the human
body temperature or temperatures of local origin such as the observatory cellar
in Paris or the highest temperature in sunshine in London. Of course also the freezing
and boiling points of water were used, but it was not self-evident that they really were universal and e.g. independent of the geographic latitude.

Anders Celsius should be recognized as the first to perform and publish careful
experiments aiming at the definition of an international temperature scale on
scientific grounds. In his Swedish
paper "Observations of two persistent
degrees on a thermometer" he reports on experiments to check that the
freezing point is independent of latitude (and also of atmospheric pressure!).
He determined the dependence of the boiling of water with atmospheric pressure
(in excellent agreement with modern data). He further gave a rule for the
determination of the boiling point if the barometric pressure deviates from a certain
standard pressure.

The position of the degree zero on a temperature scale has created a lot of
discussions. The French Réaumur scale had zero at the freezing point but as regards
other scales one placed zero outside the ordinary temperature region, thus avoiding
the mixture of positive and negative numbers. The zero could be placed at a low
temperature, a method used by the Danish astronomer Ole Rřmer and then adopted by
Fahrenheit. Celsius was accustomed to the Réaumur thermometer, but he also used a
thermometer made by the French astronomer Joseph-Nicolas Delisle with zero at the
boiling point, thus creating a reversed scale with increasing numbers for decreasing
temperatures, but avoiding negative numbers.

This reversed scale was not as awkward as we think today and it served its purpose.
The change to our modern direct scale was inevitable in the long run, however,
but there is no sense in trying to give the credit to any single person.
Linné has often been named as the inventor with reference to his famous work "Hortus Cliffortianus" of 1737, five years before Celsius' publication.
However, in studying Linné's work it is obvious that the few temperatures he mentiones
are impossible as modern Celsius' degrees. Linné met Fahrenheit in Holland and it
is clear that he used Fahrenheit temperatures when he returned home to Sweden.
Most probably Linné abandoned the Fahrenheit scale when he learned about Celsius'
research.
After the death of Celsius in 1744 thermometers with a direct scale
appeared in the meteorology reports under different names, such as 'Celsius
Novum', 'Ekström' and 'Strömer'. Linné had a great interest in
thermometers, and he ordered a thermometer with the modern scale from the
instrument maker Daniel Ekström around 1744. The scale is mentioned in a
dissertation by Samuel Naucler in 1745, but this does not indicate Linné as
the single inventor of the modern scale. Certainly the change of scale
became obvious to the Uppsala scientists in general. In an account of the
history of the thermometer in the Proceedings of the Royal Swedish Academy
of Sciences 1749, Pehr Wargentin, Secretary of the Academy of Sciences,
mentiones Celsius, his successor Strömer and the instrument maker Ekström
in connection with the direct scale. Linné is not mentioned at all. No
single person can be given the credit.

From the scientific point of view the most important contribution to the modern
temperature scale is due to Celsius because of his careful experiments on the fixed
points. The direction of the scale comes in second place, and was hardly a great intellectual achievement.